RADIOLARIA 



8l 



hand, that the Radiolaria have an alternation of generations like that 

 of the Foraminifera. 



Peculiarities of the mitoses in this group have been mentioned 

 above (pp. 25, 26). 



Symbiotic flagellates, known as "yellow cells" {Zooxanthellae, see 

 pp. 47, 50), are present in large numbers in the cytoplasm of many 

 of the Radiolaria. 



Thalassicolla {Fig. 32 A). (Suborder Spumellaria.) Skeleton absent 

 or represented by some loose siliceous spicules; one nucleus; yellow 

 cells in extracapsular protoplasm. 



-Hn. 



nu. 



Fig. 68. A, Heliosphaera inermis, x 350. After Hertwig. B, The skeleton of 

 Actinomma. After Biitschli. sk. skeleton; cps. central capsule; 7iu. nucleus. 

 The yellow cells are shown, but not labelled, in A. 



Collozoum (Fig. 32 B). As Thalassicolla, but with central capsules 

 united by their extracapsular protoplasm into a colony; and each 

 capsule contains several nuclei. 



Heliosphaera (Fig. 68 A). As Thalassicolla, but the skeleton has 

 the form of a lattice-work on the surface of the body. 



Actinomma (Fig. 68 B). As Heliosphaera, but the skeleton consists 

 of several lattice spheres, formed successively at the surface as the 

 animal grows, with radial struts joining them. Ultimately the inner- 

 most sphere may lie in the nucleus. 



Acanthometra (Fig. 69 A). (Suborder Acantharia.) A skeleton of 

 radial spicules of strontium sulphate meeting centrally in the central 

 capsule; nuclei numerous; yellow cells intracapsular. Remarkable 

 structures known as "myophrisks", surrounding the spines of this 



